delimiter

A "character" or "string" that sets bounds to an arbitrarily-sized textual object, not to be confused with a "separator" or "terminator". "To delimit" really just means "to surround" or "to enclose" (like these parentheses are doing).

A character used to indicate the beginning and end of a character string, i.e., a symbol stream, such as words, groups of words, or frames. 2. A flag that separates and organizes items of data.